This invention relates to control systems for adjusting the pitch of variable-pitch angle blades, and specifically to a closed loop control system which automatically adjusts the pitch angle of wind turbine blades to maximize generated electrical energy, and maintain stability.
Systems for controlling the pitch of variable-pitch angle blades find application, for example, in a wind turbine driving a synchronous generator. Wind turbines, in effect large windmills, rotate and produce electrical energy in response to natural wind currents. The electrical energy can be stored, used to power isolated installations directly, or fed to power utility grids for ultimate distribution.
A major problem with wind turbines is that of controlling the electrical output power and power factor in the presence of unpredictable wind gusts and turbulence, particularly when the wind turbine is driving a synchronous generator connected to a large electrical network. Wind turbulence conditions can create shaft torque fluctuations of sufficient magnitude to cause the synchronous generator to reach dangerous power levels, cause protective circuit breakers to open and disconnect from the grid.
In an attempt to overcome this problem, certain wind turbines vary the pitch angle of the blades in a manner analogous to the blade pitch control of an aircraft in response to selected operating parameters, such as wind velocity, rotor speed and output power. A representative control system of this type is disclosed in U.S. Pat. No. 4,193,005 to Kos et al. Briefly, the Kos et al. patent teaches closed loop control of rotor blade acceleration rate and deceleration rate during startup and shutdown. The control uses a single control integrator for all closed loop operating modes with a rate limiter in front of the integrator to prevent integrator over-travel. Also employed is a closed loop shaft torque control for on-line power control.
To maximize the electrical power at low wind speeds from a wind turbine using variable-pitch angle blades, it is necessary to set the pitch angle of the blades at the angle which gives maximum torque. At this angle, however, a linear control system can become unstable since it is impossible to increase the aerodynamic torque to counteract decreasing power output caused by decreasing wind speed. A wind turbine consisting of a massive blade assembly connected to a synchronous generator by a flexible shaft and having a low resonant frequency will tend to oscillate.
A typical prior art control system is shown in FIG. 1. As shown, signal 10 representing a nominal blade angle, B.sub.O, is added at a summing junction 14 to a signal 12, representing Kdt/dp, which is proportional to the rate of change of power generated by the wind turbine. Circuits to provide such signals as signal 12 are well known in the art. A signal 16 to control the pitch of the variable-pitch angle blades is provided at the output of summing junction 14. In order to get maximum power, signal 10, B.sub.O, would be set at a value which produces maximum aerodynamic torque.
The problems with such a system can be explained using FIG. 2 which shows a graph of aerodynamic torque Q.sub.A versus the pitch angle, B, of the blades. In FIG. 2, curve 114 is for a high wind speed and curve 116 is for a low wind speed. In this example, the maximum torque occurs at a particular blade angle, which in FIG. 2 is 2 degrees for both high and low wind speeds. The equation describing the dynamics of a wind turbine connected to a synchronous generator through a flexible shaft is: ##EQU1## where: I=inertia of blade assembly;
w=angular velocity of blade assembly; PA1 Q.sub.A =aerodynamic torque; PA1 .delta.=twist angle of the flexible shaft; and PA1 K=stiffness constant of the flexible shaft. PA1 B.sub.N =steady state blade angle for nominal power.
For a synchronous generator, the instantaneous electric power, P, is proportional to the shaft torque, K.delta., and the generator is forced to rotate at constant speed, W.sub.O. It follows that: ##EQU2## Equation (1) can be re-written as: ##EQU3##
A control law which effectively damps out oscillations in P is one which controls blade angle, B, so that: ##EQU4##
If the nominal operating point is at 120 on FIG. 2, then ##EQU5## where: K.sub.3 =derivative gain;
The torque, Q.sub.A, will decrease and increase as dt/dp varies up and down from zero.
To obtain the maximum power from a given low wind speed, it is necessary to set the nominal operating point at 118 in FIG. 2, which is at the peak of the torque curve where the blade angle is B.sub.O. The control law in equation (5), however, will not work effectively at this point since the torque can only decrease regardless of how B changes. Hence the prior art control system in FIG. 1 is unsatisfactory.
The present invention overcomes these disadvantages by limiting the blade angle B so that it is never less than B.sub.O. The present invention provides damping through the use of limiters so that the nominal operating point of a wind turbine can stay at its peak, for example at point 118 in FIG. 2, thereby providing maximum power.